Fly ash produced at coal fired power plants is commonly used in ready-mixed concrete as a pozzolanic admixture and for partial replacement for cement. Fly ash consists of alumino-silicate glass that reacts under the high alkaline condition in cementitious slurry to form additional cementitious compounds when the fly ash is added to the cementitious slurry. Fly ash is an essential component in high performance concrete. Fly ash contributes many beneficial characteristics to cementitious compounds including increased density, long term strength, decreased permeability, improved durability against chemical attack, and improved workability of freshly placed material.
Coal burning power stations commonly inject ammonia or ammonia based reagents into associated flue gas containing fly ash in an effort to: (1) enhance electrostatic precipitator (ESP) performance to reduce opacity and (2) remove nitrous oxide (NOx) using selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) technologies to meet NOx emission regulations. Ammonia injection into the flue gas for ESP, SCR and SNCR performance enhancement commonly results in the deposition of ammonia on the fly ash. Also, gas phase reaction of SOx and NH3 in the flue gas results in the deposition of ammonium salts on the fly ash in the form of ammonium sulfatexe2x80x94(NH4)2SO4 and ammonium bisulfatexe2x80x94NH4HSO4. In both SCR and SNCR processes, NOx is reduced using ammonia to produce nitrogen gas (N2) and water (H2O) vapor according to the following reaction:
4NO+4NH3+O2xe2x86x924N2+6H2O 
2NO2+4NH3+O2xe2x86x923N2+6H2O 
The degree of ammonia contamination in the fly ash, and associated concentration levels, vary among power plants depending on the rate of ammonia injection, the performance of SCR or SNCR process, the amount of SO3 in the flue gas and the associated operating conditions of the boiler and air pollution control devices. It has been observed that fly ash produced from high sulfur eastern bituminous coal (Class F fly ash) adsorbs more ammonia than fly ash produced from low sulfur western sub-bituminous coal (Class C fly ash). As previously mentioned, the presence of sulfur in the flue gases increases the associated deposition of ammonia in the form of (NH4)2SO4 and NH4HSO4. The high alkaline condition of Class C ash inhibits ammonia cation (NH4+) formation. Typical ammonia concentrations on fly ash, as a result of ammonia injection, ranges between 50-120 mg/kg for SCR generated fly ash, 250-600 mg/kg for SNCR generated fly ash, and 700-1200 mg/kg for ESP generated fly ash.
When ammonia-laden fly ash is used in cementitious slurry applications, the ammonium salts dissolve in water to form ammonia cations (NH4+). Under the high pH (pH greater than 12) condition created by cementitious alkali, ammonium cations (NH4+) are converted to dissolved ammonia gas (NH3). Ammonia gas evolves from the fresh cementitious slurry into the air, exposing workers. The rate of ammonia gas evolution depends on ammonia concentration, mixing intensity, exposed surface, and ambient temperature. Ammonia has no measurable effect on concrete quality (strength, permeability, etc.). Ammonia gas odors could range from mildly unpleasant to a potential health hazard. Ammonia odors are detected by the human nose at 5 to 10 ppm levels. The OSHA threshold and permissible limits are set at 25 and 35 ppm for the time weighted averagexe2x80x94eight-hour (TWA 8-hr) and the short term exposure limitxe2x80x94fifteen-minute (STEL 15-min), respectively. Ammonia gas concentration of 150-200 ppm can create a general discomfort. At concentrations between 400 and 700 ppm ammonia gas can cause pronounced irritation. At 500 ppm, and above, ammonia gas is immediately dangerous to health; at 2,000 ppm, death can occur within minutes.
Other than OSHA exposure limits, there are no regulatory, industry or ASTM standards or guidelines for acceptable levels of ammonia in fly ash. However, based on industry experience, fly ash with ammonia concentration at less than 100 mg/kg does not appear to produce a noticeable odor in ready-mix concrete. Depending on site and weather conditions, fly ash with ammonia concentration ranging between 100-200 mg/kg could result in unpleasant or unsafe concrete placement and finishing work environment. Fly ash with ammonia concentration exceeding 200 mg/kg produces unacceptable odor when used in ready-mixed concrete applications.
In addition to the risk of human exposure to ammonia gas evolving from cementitious slurry produced using ammonia-laden ash, the disposal of the ammonia-laden fly ash in landfills and ponds at coal burning power stations also creates potential risks to humans and the environment. Ammonium salt compounds in fly ash are extremely soluble. Upon contact with water, the ammonium salts leach into the water and are carried to ground water and nearby rivers and streams causing potential environmental damage such as ground water contamination, fish kill and eutrophication. Ammonia gas could also evolve upon wetting of alkaline fly ashes, such as those generated from the combustion of western sub-bituminous coal. Water conditioning and wet disposal of alkaline, ammonia-laden, fly ash exposes power plant workers to ammonia gas.
The present invention relates to the addition of a chemical oxidizing agent to dry fly ash containing concentrations of ammonia. The chemical can be added and blended with the dry fly ash at any point between the fly ash collection system at the power plant and final delivery to the ready-mixed customer, or at the point of use at the ready-mixed customer site. The pre-blended chemical oxidizing agent does not react with ammonia in the dry fly ash; the chemical oxidizing agent is released during the wet slurry mixing process. Once the ammonia-laden fly ash is introduced in the cementitious slurry, ammonium salts from the ammonia-laden fly ash dissolve. The high alkaline (high pH) condition of the cementitious slurry converts the ammonium cations (NH4+) to dissolved ammonia gas (NH3). Without the chemical oxidizing agent, ammonia gas (NH3) evolves from the cementitious slurry during mixing, transportation, pouring and placement.
More specifically, this invention relates to pre-treated ammonia-laden fly ash and to methods of treating the ammonia-laden fly ash. Addition of the chemical oxidizing agent with the dry ammonia-laden fly ash prior to incorporating the fly ash into cementitious slurries results in chemical conversion, via oxidation, of ammonia into harmless products. Thereby, the exposure risk of the ammonia gases (NH3) is limited.
The preferred chemical treatment reagents are strong oxidizers such as hypochlorites (OClxe2x88x92) commonly found in the form of Ca(OCl)2, NaOCl, LiOCl, trichloro-s-triazinetrione (trichlor), etc. and are added to the ammonia-laden fly ash. Preferably, the oxidizers are added in dry form to the fly ash, but it is also possible to spray a dilute solution (containing up to about 30% oxidizer) onto the fly ash. At present, calcium hypochlorite is preferred. The reagent is activated upon water addition and reacts with dissolved ammonia in the ash or concrete slurry to form primarily monochloramine (NH2Cl). An overdose of the hypochlorite reagent would further oxidize monochloramine to form nitrogen gas (NH2) and chlorides.
As used herein, the phrase hypochlorite containing oxidizer is used to denote compounds that include the hypochlorite moiety or form such moiety upon addition of water. For example, the trichor compound forms hypochlorous acid and cyanuric acid upon water addition. At elevated pHs, the hypochlorous acid ionizes to the hypochlorite ion.
The basic aqueous phase ammonia oxidation reaction using hypochlorite is as follows:
NH4++OClxe2x88x92xe2x86x92NH2Cl+H2O 
The rate of ammonia oxidation by hypochlorite depends upon pH, temperature, time, initial dosage and the presence of competing reducing agents. The pH condition of this reaction in Portland cement based concrete and mortar is governed by the presence of alkali from the associated cement hydration. The expected cementitious slurry pH is between 12 and 14. The temperature of freshly mixed concrete tends to be slightly warmer than the ambient temperature as a result of the heat of hydration. The optimum concrete temperature is in the range of 10 to 15xc2x0 C. (50 to 60xc2x0 F.), or lower for massive concrete pours, to avoid thermal cracking. Concrete temperature should not exceed 33xc2x0 C. (90xc2x0 F.). Time of reaction is also governed by conventional and standard concrete practices namely mixing, handling and placing guidelines. Ready-mixed concrete batches are mixed for at least 5 to 10 minutes. ASTM C94 requires the concrete to be placed within 90 minutes of mixing. The ammonia, in ammonia-laden fly ash and concrete mixtures, represents the most readily available reducing agent to react with hypochlorite. The chloramine forming reaction of ammonia and hypochlorite in water are 99% complete within a few minutes. Theoretically, a 1:1 molar ratio of hypochlorite to ammonia (Cl:N) is needed to produce monochloramine. Further increases in the molar ratio of Cl:N result in further oxidation and formation of nitrogen gas and chloride salts.
Fly ash and Portland cement are produced under strong oxidizing conditions in boilers and kilns. Therefore, the fly ash and Portland cement tend to be void of any reducing agents during production; ammonia is introduced into the fly ash in post-combustion air pollution control devices. Concrete aggregates (i.e., sand and gravel) should be free of deleterious organic substances that could otherwise exert a demand on hypochlorite. The presence of some organic admixtures in concrete (e.g., air entraining and water reducing agents) do not present any measurable demand on hypochlorite since ammonia is a strong reducing agent and the desired product, monochloramine, is also a reducing agent.
Other advantages and benefits will be apparent to one skilled in the art when reviewing the specification in combination with the drawings as described herein.